The present invention relates to a method and an apparatus for measuring insulation resistances and, particularly, to a method and an apparatus for continuously measuring insulation resistances of a non-grounded DC circuit including a rectified electric power source and of load means connected to that DC circuit.
Insulation control of low-voltage electric machines such as, for example, large DC rotary machines and the like is essential for securing safety and maintaining safe operation in, for example, manufacturing plants in which such machines are used. For this purpose, it is desirable that information on the insulating condition of each such machine is collected continuously and automatically.
Heretofore, since measurement of the insulating condition of such machines for insulation control was performed by maintenance workers who checked each machine with a megaohmmeter each time it was necessary after the operation of the machine was finished, opportunities for such measurement were limited, each measurement required many steps, security of the maintenance workers was insufficient, and continuous measurement during operation of the machines was impossible at all.
We previously proposed apparatuses capable of measuring the insulation resistances of a non-grounded DC circuit and the load means connected thereto during the operation thereof which was heretofore impossible by megaohmmeter. Namely, Japanese Pat. No. 76073/77 discloses means for measuring three voltages across the positive line and the negative line and across each of them and the ground of a non-grounded DC circuit in an operative condition and for calculating the three insulation resistance values across said positive and negative lines and across each of these two lines and the ground directly from the measured voltage values, means for calculating the variation rate of the voltage across the positive and negative lines from the measured voltages and for thereafter calculating the insulation resistance value across said positive and negative lines from this variation rate and the measured voltage values, and means for calculating the ratio of the insulation resistance value calculated for each measurement to the value, the preceding measurement. Japanese Pat. No. 91475/77 discloses means, as in Japanese Pat. No. 76073/77, for measuring three voltages successively and calculating a voltage ratio for each measured voltage and further calculating the insulation resistance value across the positive line and the negative line from these voltage ratios, and means for calculating the ratio of the voltage ratio of each measurement to the voltage ratio of the preceding measurement. Japanese Pat. No. 91476/77 discloses means for measuring three instantaneous voltage values across the positive line and the negative line and across each of these and the ground of a non-grounded DC circuit in an operative condition, calculating the average variation value and the average value of the measured instantaneous voltage values, and calculating the insulation resistance values across the positive and negative lines, across a rotary machine between these two lines and the ground, and across each of these two lines and the ground directly from the average variation value and the average value and the measured instantaneous voltage values, means for calculating respective voltage ratios from the average variation value, the average value and the three instantaneous voltage values and calculating thereafter insulation resistance values as above from these voltage ratios, and means for calculating the ratio of the voltage ratio calculated for each measurement to the voltage ratio of the preceding measurement. These insulation resistance measuring apparatuses according to prior art are relatively complicated in the principle for calculation of the insulation resistances and require many voltages to be measured. Accordingly, these prior apparatuses are inevitably of a complicated construction requiring complicated operation which tends to result in an undesirable decline in reliability, increase in cost and complexity of maintenance.
Accordingly, a principal object of the present invention is to provide a relatively simple method for obtaining various required insulation resistance values by continuous measurement of only two voltages.
Another object of the present invention is to provide a method having a reference for evaluating various calculated insulation resistance values and capable of monitoring variation of each insulation resistance value and of giving an alarm, when necessary.
A further object of the present invention is to provide an apparatus for practicing said objects.